The present invention relates to a particle sensor for detecting smoke particles or the like, and more particularly to such a particle sensor having an output adjustment capability.
Particle sensors have been widely utilized in the art for monitoring the amount of particles such as smoke particles in an environment in order to determine the criticality level of the particle density. The particle sensor is normally designed to include a photo-detector which provides an output voltage proportional to the amount of the particles carried on the air being monitored. A light emitter is utilized in association with the photo-detector to project a light beam into a detection chamber for giving the scattered light due to the presence of the particles in the chamber. It is this scattered light that is collected by the photo-detector which, in turn, provides the output voltage indicative of the amount of the particles present in the chamber. A gain of the output voltage is then processed in order to satisfy a predetermined or regulation relationship between the output voltage and a particle density. Further, in order to cancel a background noise, i.e., a background voltage such as resulting from a stray light received by the photo-sensor, a suitable offset voltage reflecting the background voltage is combined with the output voltage to give a sensor output truly indicative of the amount or density of the particles. The gain control and the offset voltage are each realized by a mechanical variable resistor. Although the large number of the components forming the sensor can be easily assembled into an integrated circuit of a compact structure, the mechanical variable resistors which are inherently bulky have to be external to the integrated circuit and become hindrance to making the whole sensor structure compact. Also, since the mechanical variable resistors are external to the integrated circuit, they may become sources of noise for the integrated circuit, lowering reliability of the detector. Further, the mechanical variable resistor is not suitable for remote control adjustment of resistance because of the remote control adjustment requires a complicated means of processing visual image of an adjustor dial of the mechanical variable resistor to estimate a current resistance and further actuating the adjustor dial.
The present invention has been accomplished in view of the above problems to provide a particle sensor which is compact, reliable in operation, and easy for adjustment. The particle sensor in accordance with the present invention comprises a detector proving an output voltage which is proportional to the amount of particles carried on a medium such as the air being detected, a gain controller adjusting the output voltage received from the detector to provide an adjusted output voltage, and an offset voltage adjustor providing an adjustable offset voltage indicative of a background voltage or noise. The offset voltage is combined with the adjusted output voltage to provide a sensor output which. satisfies a predetermined relationship between particle density and the sensor output. The gain controller includes a gain resistor network which gives a variable resistance in order to vary the adjusted output voltage, and the offset voltage adjustor includes an offset resistor network which gives a variable resistance in order to adjust the offset voltage. Either one or both of the gain resistor network and the offset resistor network comprise a plurality of digitally controllable switches and a plurality of resistors so as to give the variable resistance varying by conduction of a suitable combination of the switches. Also included in the sensor is a memory module which stores an instruction data designating which one or more of the switches are to be made conductive, and a memory interface which transfers the instruction data from the memory module to at least one of the gain resistor network and the offset resistor network. With this arrangement, the gain resistor network and the offset resistor network can be assembled together with the gain controller and the offset voltage adjustor into a single compact integrated structure. Thus, the whole sensor can be made compact and be assembled by a reduced number of parts to lower a manufacturing cost. Also with the inclusion of the resistor networks in the integrated circuit, they can be less susceptible to an external noise so as to make the sensor reliable. Further, since the resistor networks is realized as a digitally adjustable resistor network, adjustment of the resistance can be made easy simply by electronically varying the instruction data.
The memory module utilized in the present invention includes a normal type which consist of a non-volatile memory such as EEPROM for storing the instruction data, and an intelligent type composed of a like non-volatile memory storing the instruction data and a microcomputer capable of writing the instruction data. In order to make the particle sensor compatible with the two types of the memory modules, the memory interface is designed to have a memory controller and a selector. The memory controller sends a first clock signal and read signal for reading the instruction data directly from the non-volatile memory in accordance with the first clock signal and transferring the instruction data to the resistor networks. The selector is configured to have inputs respectively adapted to receive the first clock signal from the memory controller and a second clock signal, and to select one of the first and second clock signals. The second clock signal is supplied from other than the memory controller, i.e., from the microcomputer and is utilized to read the instruction data from the non-volatile memory under the control of the microcomputer and to transfer the instruction data to the resistor networks. Thus, the sensor can operate with either of two types of the memory modules simply by selecting the clock signal at the selector, which is therefore another object of the present invention.
In this connection, the memory interface may include a shift-register which receives the instruction data from the non-volatile memory either by way of the microcomputer or directly from the non-volatile memory, and transfers the instruction data to the resistor networks. The shift-register is connected to the selector to receive the selected one of the first and second clock signal and is connected to receive the instruction data through a data channel. When the normal memory module is utilized, the data channel is connected to receive the instruction data directly from the memory. For the intelligent memory module, the data channel is connected to the microcomputer to receive the instruction data through the microcomputer. The instruction data is transmitted in accordance with the selected one of the first and second clock signal into the shift-register to be subsequently delivered to the resistor networks. With the use of the shift-register, it is possible to check validity of the instruction data for increased reliability of the sensor output.
When the intelligent memory module is utilized, the instruction data is preferred to have a data structure composed of the following four separate data.
(1) A gain value data having plural bits each designating a conduction state of the corresponding one of the switches included in the gain resistor network,
(2) A reverse gain value data having reversed bits of the gain value data;
(3) An offset value data having plural bits each designating a conduction state of the corresponding one of the switches included in the offset resistor network; and
(4) A reverse offset value data having reversed bits of the offset value data.
In this connection, the memory interface includes a data validation unit which fetches the instruction data from the shift-register to compare the bits of the gain value data with the corresponding reversed bits, and compare the bits of the offset value data with the corresponding reversed bits in order to verify the gain value data and the offset value data, and provides an error signal when any one of the data is not verified. In response to the error signal, the microcomputer acts to retransmit the instruction data from the memory to the shift-register. Thus, even when the instruction data should become erroneous due to the influence of transient noises, the valid instruction data can replace for the erroneous data to keep providing the reliable sensor output, which is therefore a further object of the present invention.
When the normal memory module is utilized, the memory controller operates to fetch the instruction data periodically from the non-volatile memory for transfer to the resistor networks so as to keep transferring the valid instruction data, thereby ensuring reliable sensor output while avoiding temporary errors which might occur due to transient noises.
Further, the memory interface may include a write interface which accepts a write signal from an external writing device and enables the writing device to write the instruction data in the memory means for facilitating the adjustment of the resistance.
These and still other objects and advantageous features of the present invention will become more apparent from the following description of the embodiments when taken in conjunction with the attached drawings.